With regard to an insulated electric wire which is constituted by coating a conductor with an insulating resin, that where PVC is used as a coating material has been widely known. The above-mentioned insulated electric wire has excellent characteristics such as resistance against dielectric breakdown and flexibility due to excellent insulation and flexibility of a soft vinyl chloride resin used as a coating material and, moreover, it has good water resistance and oil resistance whereupon its has been frequently used for household use and for automobiles.
With respect to the insulated electric wire using PVC however, development for insulated electric wire using no PVC has been developed from the consideration on environmental load.
As to the above-mentioned insulated electric wire using no PVC, various ones using polyolefin resin having excellent characteristics in insulation, water resistance and oil resistance have been proposed.
Among them, that where soft polyolefin mainly comprising an elastomer containing a lot of ethylene unit for achieving flexibility is used as a coating material is the mainstream.
However, the above-mentioned soft polyolefin contains a lot of ethylene unit and, therefore, there is a problem that heat resistance lowers and resistance of the resulting insulated electric wire against deformation by heating is deteriorated. There is another problem that, due to insufficient rubbery characteristic of the resin, working ability upon connection of the insulated electric wire using such a soft polyolefin is deteriorated.
Further, in the above-mentioned soft polyolefin used for the above-mentioned insulated electric wire, an inorganic filler is usually compounded for a purpose of enhancing the mechanical strength of the resulting insulated electric wire and giving flame retardant property. However, the insulated electric wire prepared using the resin composition compounded with the inorganic filler is apt to generate microcrazes and has poor resistance against whitening by bending. In addition, the above-mentioned whitening phenomenon lowers the discriminating power of colored wire and causes thermal deterioration in a heat-resisting use for a long period.
Further, the above-mentioned soft polyolefin has another problem of lowering of abrasion resistance by compounding of the inorganic filler. Especially when the modulus of elasticity in flexure of the soft polyolefin is adjusted to not more than 2,000 MPa for achieving the flexibility, scraping property of the insulated electric wire significantly lowers.
In order to improve the abrasion resistance of molded products comprising soft polyolefin compounded with an inorganic filler, there has been proposed a resin composition where the soft polyolefin resin is modified with an acid so that an organic base is bonded. However, the actual current situation is that an effect of improving the scraping property is insufficient and that resistance against deformation by heating is not achieved.
On the other hand, in the manufacturing steps of insulated electric wire, the soft polyolefin and the above-mentioned PVC usually have a big swell ratio and, in molding an insulated electric wire by means of an extrusion molding, a melt fracture phenomenon is apt to be generated in a region of a low shear rate and the productivity is poor.
Accordingly, an object of the present invention is that, in an insulated electric wire where a soft polyolefin composition containing the above-mentioned inorganic filler is used as a coating material, there is provided an insulated electric wire in which good insulation, water resistance and oil resistance of the polyolefin are retained, bending property, resistance against scrape, resistance against deformation by heating and resistance against whitening by bending are still improved and productivity is good as well.